That unit only extends a normal PSTN line to a remote location with the two unit 'paired' - not part of a DECT system.
Somewhere I have the sort of unit that is being asked for. It was produced by BT for linking to their Diverse base stations about 15 years ago. You plugged a DTMF 2 wire basic telephone into it - or you could extend the wiring to a remote master socket. The unit had an 'extension' number as part of the DECT phone system. Can't recall seeing a similar unit since.
You'll find the manual
here